Radar response system

Abstract

A covert radar response system suitable for use with an illuminating coherent radar system having a moving target indicator subsystem shifts the phase and/or frequency of a retroreflected beam to transmit a message to the illuminating system that is not detectable by other observers in the vicinity.

What is claimed is: 
     
       1. A radar system comprising an antenna; a transmit chain of elements connected to said antenna;   a receiver chain of elements connected to said antenna for detecting a radar return signal having a return amplitude, return phase and return frequency;   timing and control means for directing power to said antenna and for receiving signals collected by said antenna;   an MTI target detector connected to said receiver chain for detecting frequency and/or phase shifts in a return signal returned by a target; and   a speed/pattern indicator connected to said MTI target indicator for processing signals therefrom to extract information representing the speed of said target, characterized in that:   said speed/pattern indicator includes means for recognizing a predetermined speed/pattern signal representing a message sent from said target, whereby said target may communicate with said radar system by means of modulation of the phase or frequency of a radar target return to impress thereon a predetermined pattern representing a message, without substantially patterning said amplitude of said radar target return.   
     
     
       2. A system according to claim 1, further characterized in that said means for recognizing a pattern includes phase means for detecting modulations in phase of said radar target return. 
     
     
       3. A system according to claim 1, further characterized in that said means for recognizing a pattern includes frequency means for detecting modulations in frequency of said radar target return. 
     
     
       4. A system for responding to an interrogating radar signal having an input phase, input amplitude envelope, and input frequency comprising an antenna; means for modulating said interrogating signal to generate a return radar signal differing from said interrogating radar signal in a pattern of phase shifts relative to said input phase, but having said input frequency and input amplitude envelope, and   means for directing said return radar signal to said antenna, whereby said interrogating radar signal is returned with said predetermined pattern of phase shifts.   
     
     
       5. A system according to claim 4, in which said means for modulating said interrogating radar signal comprises controllable impedance means for imposing a controllable phase shift on said interrogating radar signal and reflecting means for reflecting received interrogating power back through said antenna. 
     
     
       6. A system for responding to an input radar signal having an input phase input and amplitude envelope and input frequency comprising a first antenna pointing in a predetermined direction; means for modulating said input radar signal to generate a return radar signal having substantially said input amplitude envelope in a reflected return radar signal, but differing from said input radar signal in a pattern of frequency shifts relative to said input frequency, and   means for directing said return radar signal to a second antenna pointing in said predetermined direction whereby said system responds with a return radar signal substantially the same as a reflected return signal, but differing from a reflected return signal by said pattern of frequency shifts.   
     
     
       7. A system according to claim 6, in which said means for modulating said interrogating radar signal comprises means for generating a predetermined pattern of intermediate frequency signals and combining said intermediate signals with said interrogating radar signal to generate a radar return signal related to said interrogating radar signal and to said intermediate signals. 
     
     
       8. A system for responding to an interrogating radar signal having an input phase, input amplitude envelope, and input frequency comprising an antenna; passive means for modulating said interrogating signal to generate a return radar signal differing from said interrogating radar signal in a pattern of phase shifts relative to said input phase, but having said input frequency and input amplitude envelope, and   means for directing said return radar signal to said antenna, whereby said interrogating radar signal is returned with said predetermined pattern of phase shifts.